In any vehicle sensing system of the type under consideration, there are two basic components. These are a wire loop, which is embedded in the roadway, and an electronic control circuit which is connected to the loop, and which responds to changes in the inductance of the loop to sense the presence of vehicles in the roadway. The control circuit serves to operate an appropriate traffic signal. When a failure occurs in such a system, it is normally difficult to determine whether the failure occurred in the circuit of the embedded loop, or in the control circuit. The monitoring system of the present invention provides an instant indication in the event of such a failure, as to whether or not the failure occurred in the loop circuit.
The monitoring system of the invention finds particular, although not exclusive, utility in conjunction with vehicle detection systems of the type described and claimed in copending applications Ser. Nos. 364,442 and 443,299, both of which were filed in the name of the present inventor and which have now issued as U.S. Pat. Nos. 3,875,555 and 3,873,964. In the systems described in the copending applications, an embedded loop and lead-in circuit controls a loop oscillator which must function continuously under normal conditions. In such systems, should the loop oscillator cease oscillating, a failure condition exists which can usually be assumed to be in the loop or in its lead-in circuit. The system of the present invention senses the loop signal output of he loop oscillator, and it serves to indicate a failure condition in the loop or lead-in circuit if, for any reason, the loop oscillator ceases to oscillate.